Heavy oils are defined as crude oils whose API gravity is below 20. These oils, the world reserves of which are of the same order as for conventional oils, are characterized by a high asphaltene content and by a high viscosity that can reach up to a million centipoises at reservoir temperature. Their transportation by pipeline is therefore much more difficult than in the case of conventional crudes. Heavy crude pipeline transportation implies that the viscosity is sufficiently low considering the dimension of the transportation lines and the power of the pumping installations, selected in accordance with the economic optimum.
There are various methods known to the man skilled in the art that allow heavy oil pipeline transportation. These methods are, for example, heating, dilution, bringing into aqueous emulsion, core annular flow, or partial crude refining on the production site before transportation.
Heating is an effective way of reducing notably the viscosity of heavy oils. However, depending on the characteristics of the crude to be transported, it may be necessary to bring the fluid to relatively high temperatures, sometimes above 100° C., to obtain a viscosity compatible with industrial plants. Furthermore, it is important to maintain the temperature of the fluid at this level all along the line, which implies thermal insulation of the lines and sometimes installation of heating units combined with the pumping installations.
Emulsification of crude in water is also currently used. In this technique, the crude is transported in form of fine droplets in a continuous phase mainly consisting of water. In order to guarantee emulsion stability all along the pipeline, it is necessary to add judiciously selected surfactants to the water. These surfactants must also allow, in a simple manner, both inversion of the emulsion upon arrival at the refinery and recovery of the anhydrous crude, and treatment of the polluted water.
Core annular flow consists in transporting the crude surrounded by a water film. This is the most effective method for reducing pressure drops, which are almost comparable to those obtained with water. This technique is for example described in patent U.S. Pat. No. 4,753,261. However, this method involves difficulties related to the flow stability, fouling of the pipeline walls in the course of time and notably restarting difficulties in case of non programmed production stop, which is why this transportation mode has not been used much up to now.
Another method that can be considered for bringing the viscosity of a crude to a value compatible with pipeline transportation is partial refining on the production site. An example is given in patent U.S. Pat. No. 5,110,447. This method requires considerable investments and high operating costs due to the increase in the number of visbreaking units on the site.
In order to reduce the viscosity of heavy oils, they are commonly diluted by means of solvents. The solvents used are hydrocarbon cuts such as condensates or naphtha. This method is based on the fact that the viscosity of heavy crudes is greatly reduced when adding a solvent of low viscosity. It is generally admitted that, in order to obtain a sufficient viscosity reduction to allow pipeline transportation of a heavy oil, the amount of light solvent to be added ranges between 10 and 50% by volume. When this method is used, it most often comprises a second pipeline allowing to recycle the solvent after distillation separation at the refinery. This method can be regarded as the most effective for heavy crude transportation. Despite considerable investment, it allows oil to be transported without particular risks, even in case of prolonged production stop. Furthermore, diluting the crude facilitates certain operations such as separation of the production water. However, the volume to be transported is increased, and the cost of the solvent and of its possible separation from the crude in order to recycle it is not insignificant.
One possible improvement to the dilution of heavy crudes consists in improving the method so as to obtain the viscosity required for pipeline transportation using a lower volume of solvent.